A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include an Ethernet PON (EPON), a Gigabit PON (GPON), a 10-Gigabit (XGPON), and others.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes N ONUs 120-1 through 120-N (collectively known as ONUs 120) coupled to an OLT 130 via a passive optical splitter 140. In a GPON, for example, traffic data transmission is achieved using GPON encapsulation method (GEM) over two optical wavelengths, one for the downstream direction and another for the upstream direction. Downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to pre-assigned labels (e.g., GEM port-IDs in a GPON). The splitter 140 is a 1 to N splitter, i.e., capable of distributing traffic between a single OLT 130 and N ONUs 120. In most PON architectures, the upstream transmission is shared between the ONUs 120 in a TDMA based access, controlled by the OLT 130.
In order to provide reliable operation of the PON, there is a need to identify faults that occur on the optical fibers and/or optical components of the PON, for example, detection of breaks or of major attenuation, due to a bent fiber, dirty connectors, and so on. Additionally, in order to allow repairing of a faulty optical fiber, there is a need to locate the exact location of a fault for a faster, more efficient network repair.
Optical faults and their location in the PON can be detected using optical time-domain reflectometers (OTDRs). The principle of an OTDR includes injecting, at one end of the fiber, a series of optical pulses into the fiber under test and also extracting from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time and may be plotted as a function of fiber length. The results may be analyzed to estimate the fiber's length, the overall attenuation, to locate faults, such as breaks, and to measure optical return loss.
OTDR measurement techniques of the PON include “out of band”, “in band”, and different wavelengths. Out-of-band testing requires stopping the normal operation of the network and verifying the fiber using external OTDR tools. This can be performed using, for example, wavelengths and test pulses that are separate and independent from all other wavelengths used to carry customer service traffic.
The in-band OTDR testing may be performed when the network is operational. However, such a testing requires dedicated OTDR testing signals. The OTDR testing signals utilized in conventional in-band OTDR solutions are either AM modulated or FM modulated. As such they can be transmitted during a test period of the PON, during which time data signals are not transmitted to the ONUs. Other OTDR solutions utilize a dedicated upstream wavelength for measuring reflection from the fiber. However, such solutions require an additional transceiver.
It would be therefore advantageous to provide a solution for performing reflection analysis in a PON while overcoming the deficiencies of prior art testing techniques.